In the early 1970's, the federal government enacted the Federal Motor Vehicle Safety Standard 214 (FMVSS 214) to specify side door strength requirements for vehicles to minimize injury during side collisions. The structural reinforcements added to the door of the vehicles are commonly called impact beams, intrusion beams, guard beams, or door beams. Vehicle impact door beams have been provided in a variety of cross sectional configurations, including I-beam, hat section, and more recently, tubular. Each of these has effectiveness, if properly designed, with the tubular beam considered by the inventors herein to have the following advantages:
1) Shortened development time and cost, since tubing thickness can be readily selected in any of a number of outside diameters, and designs can be fine tuned in the development stage. A significant advantage is that extensions and weld fixtures do not have to be modified to accommodate the various beam configurations.
2) Tubing can be nested by laser cutting to further reduce weight and cost, i.e., cutting of the tube at one location produces two ends of two different tubes.
3) Tubing can be laser cut to clear latch rods, outer panels, etc., i.e., fitting within the door without incurring cost penalties.
4) Tubing has a higher torsional resistance than open hat sections. This becomes particularly important on plastic doors where torsional rigidity is critical.
5) Tubing fits in extensions with semi-cylindrical cavities, conveniently allowing for anchoring extensions to be located in the correct angle without affecting the welding of the tube to the extension. This allows for tighter twist tolerances to be met and is considered essential when mounting extensions such as paddles are not parallel to each other.
6) Extension standardization easily achieved from end to end, and car line to car line, due to the semi-cylindrical tube pocket. Extensions can be rotated into position without changing extension surfaces.
In prior U.S. Pat. Nos. 4,636,608 and 4,708,390 is set forth a tubular door beam structure having specially formed tapered ends. Subsequently, a modified taper configuration as set forth in FIG. 4 was used. The taper cuts on each end of the present tubular beam, a) accommodate the vehicle door curvature while readily allowing the beam to have its ends attached to anchor plates as by welding, b) enable the beams to be nestable, and c) cause the beam to be lighter in weight, among other factors. The length of the taper cut on both ends has in the past been primarily determined by stock length and beam length, in efforts to avoid excess scrap. Performance has been based on door stiffness and strength. Since the greatest moment would be at the center of the beam, this would be expected to be the point at which yielding first occurs. Door strength tests are conducted with a ram loaded at the beam center. Determining the length of the cut by stock length and beam length can lead to excessive taper length, and result in a tendency to fail, i.e., buckle, in the taper itself rather than in the main central portion of the beam. It has now been found that in some portions of prior taper cuts, the moment capacity M.sub.C dips below the bending moment M.sub.B so that the moment ratio M.sub.R is less than one. One way to attempt to prevent this is to make the taper cut considerably shorter. However, this can result in structures having a large excess of capacity at parts of the taper cuts. This excess capacity results in use of additional costly material, with resultant added weight. It has been determined that what is needed is a taper cut with moment capacity in all portions greater than the bending moment, yet eliminating excess capacity. This will streamline the beam and also save on material costs. In other words, it would be desirable to be able to provide an improved version of the basic tubular beam configuration set forth in the prior art wherein there is adequate but not excessive strength in the taper cuts, and also being nestable.